Reservoir monitoring refers to the gathering and analysis of information from fluids produced during the treatment of a subterranean formation penetrated by a well.
In multi-zone fracturing operations, reservoir monitoring has been used to assess the productivity of zones from which fluids are being produced. In addition, reservoir monitoring is often used to prevent the inadvertent flow of fluids into a non-productive zone or a zone of diminished interest. Selective stimulation becomes pronounced as the life of the well declines and productivity of the well decreases. Monitoring of produced fluids is also often used to determine water saturation levels in the well. In addition, reservoir monitoring provides an understanding of the dynamics of hydraulic fracture placement and subsequent fluid flowback and clean up.
Further, reservoir monitoring is used to determine in-situ downhole conditions. For example, the measurement of the pH of produced fluids may be indicative scale build-up within the well (high pH) and corrosion of wellbore equipment (low pH). Conventionally, the pH of the formation fluid is determined by obtaining a sample of the formation fluid and analyzing the sample in a laboratory. However, as the formation fluid is brought from formation conditions (e.g., high temperature high pressure conditions), acid gases and salts may come out of solution, irreversibly changing the pH of the sample. Thus the analyzed sample may not be an accurate representation of the pH of the formation fluid at formation conditions.
In the past, produced fluids have been monitored and downhole conditions have been assessed by the use of nanosize chemicals. Such monitoring methods have included the introduction of dyes (e.g., phenol red, methylene blue, and/or cresol red) as nanosize chemicals into the formation and correlating the pH of the formation fluid to the color of the dye. However, most dyes are chemically unstable under formation conditions and are also inaccurate. For example, some dyes are sensitive only within a narrow pH.
Flurophores (compounds that can re-emit light upon light excitation) have also been used as nanosize chemicals and particles. The presence of nanoparticles in produced fluids may be determined by optical spectroscopy (absorbance, fluorescence and phosphorescence). In the past, fluorophores have included organic molecules and rare-earth complexes that are toxic and/or radioactive and thus contaminate the formation (e.g., aquifers located in the subterranean formation). Further, fluorophores often are decomposed at downhole conditions and thus are the subject of photobleaching (i.e., the photochemical alteration of the fluorophore such that it becomes permanently unable to fluoresce) and photo blinking (i.e., fluorescence intermittency). The use of fluorophores are thus only useful for a short time following placement of the nanoparticle within the well and thus are restricted to near-wellbore production activity.
Alternative methods of monitoring produced fluids in well treatment applications are therefore desired.